This invention relates to electrostatographic reproduction machines, and more particularly to such a machine including a plurality of selectable fusing assemblies for increasing image quality and insuring machine reliability and productivity.
Generally, the process of electrostatographic reproduction, as practiced in electrostatographic reproduction machines, includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. A charged portion of the photoconductive surface is exposed at an exposure station to a light image of an original document to be reproduced. Typically, an original document to be reproduced is placed in registration, either manually or by means of an automatic document handler, on a platen for such exposure.
Exposing an image of an original document as such at the exposure station, records an electrostatic latent image of the original image onto the photoconductive member. The recorded latent image is subsequently developed using a development apparatus by bringing a charged dry or liquid developer material into contact with the latent image. Two component and single component developer materials are commonly used. A typical two-component dry developer material has magnetic carrier granules with fusible toner material adhering triobelectrically thereto. A single component dry developer material typically comprising toner material only can also be used. The toner image formed by such development is subsequently transferred at a transfer station onto a copy sheet fed to such transfer station, and on which the toner material image is then heated and permanently fused, as by heat and pressure, so as to form a "hardcopy" of the original image.
In order to permanently fix or fuse toner material onto a copy sheet or support member by heat and pressure rolls, it is ordinarily necessary to apply pressure and elevate the temperature of the toner to a point at which the toner material becomes tacky and coalesces. This action causes the toner to flow to some extent into the fibers or pores of the support medium (typically paper). Thereafter, as the toner material cools, solidification of the toner material occurs, causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fusing toner images, black and white or multicolor, onto a support member is old and well known.
It has been found that fusing assemblies, that include a fuser roll and a pressure roll and are used in multicolor machines to fuse multicolor toner images, intrinsically have shorter fuser roll lives as compared to black and white fusing assemblies operating at comparable copy rates. In addition, color image fusing assemblies typically produce borderline gloss and fusing level acceptability in image quality. In particular, it has been found that the faster the speed of the fusing assembly, the less acceptable the performance and quality. For example, attempting to increase the speed of a fusing assembly or fuser from 65 cpm (copies per minute) for which it is originally designed, to 72 cpm rendered its performance less than acceptable. However, it was found that if the same fuser (originally designed for 65 cpm) was run instead at a lower speed of say 36 ppm, the performance was significantly improved and the range of gloss output increased, apparently because thermal stresses on it were reduced.